IP Telephony
Nearlly all companies are trying to deploy IP Telephony because of it's simplisty, amazing advantagies and it's consider to be an important part in smart building or smart home.
There many protocols in IP Telephony systems, SIP, Skinny.......
some system create there own protocal like cisco using only skinny (the new version 5 will support SIP)
Foundary support all protocol
Nortel, Avay and Simens is also good
To look deep in these companies (cisco, Foundary, Nortel, Avay and Simens), google them
Best Regards,